Last edited 11 Dec 2020

Designing HVAC to resist harmful pathogenic microorganisms (bacteria and viruses such as influenza and 2019-nCov)


[edit] Introduction

The presence of airborne particles like dust, harmful pathogenic microorganisms like bacteria, viruses like influenza and 2019-nCov also known as Coronavirus is inevitable in occupied closed spaces/buildings. Heating Ventilation and Air Conditioning (HVAC) plays a vital role in making those buildings habitable. Other than providing comfort, it also affects the wellbeing of occupants by maintaining a healthy indoor environment.

One of the benefits of a well-designed HVAC system is control over harmful pathogenic microorganisms such as bacteria and viruses. Because of their harmful effects on health, wellbeing and indoor environments building owners must ensure these are kept under control. There are many methods to control those bacteria and viruses however this article will be discussing the following methods:

It is possible to use these methods alone or in combination depending upon project requirements.

[edit] Relative humidity control

Relative humidity is the percentage of water in the air relative to the maximum amount the air can hold.

How does humidity control reduce the spread of viral infections?

The viruses responsible for the most common cold, influenza and 2019-nCoV spread more easily when the air is dry as viruses and airborne particles stay longer in the air when there is low humidity.

As the people in the Middle East spend most of their time indoors in closed spaces with the air conditioning working, viruses spread more easily.

The Centre of Disease Control and Prevention study states that “At humidity levels of 23 percent, 70 to 77 percent of the flu virus particles were still able to cause an infection an hour after the coughing simulation. But when humidity levels were raised to 43 percent, just 14 percent of the virus particles had the ability to infect. Most of the flu particles became inactive 15 minutes after they were released into the humid air. "The virus just falls apart," at high humidity levels.”

Though raising the humidity of the whole building is a challenge, it is highly recommended for the hospitals, clinics and the places where there is a high risk of flu transmission to consider increasing their humidity levels (30% - 40 %).

However, the humidity levels should not be too high because then molds etc. may start to grow.

[edit] Increased ventilation

Ventilation is the process by which stale air is removed and clean air is provided to a space. This may be accomplished by either natural or mechanical means.

As published by the Franunhofer Institute for Building Physics, people in industrialised countries spend 90% of their time indoors and as most of the air conditioning in those countries are of a recirculated type (with 20% fresh air addition only) to conserve energy, this results in an increased chance of airborne particles (harmful pathogenic microorganisms such as bacteria and viruses) staying longer in the space.

Ventilation can reduce the concentration of airborne pathogens through removing or diluting airborne droplet nuclei. A higher ventilation rate can provide a higher dilution capability and consequently, potentially reduce the risk of airborne infections.

This can be addressed by introducing additional ventilation (~40% instead of 20%) for high-risk areas. This is best done at the design stage, as once the building is constructed there is not much that can be done without breaking concrete and drilling holes in the walls and slab for duct installation.

[edit] Introducing High Efficiency (HEPA) Filters

HEPA filters are high-efficiency particulate absorbing filters. A filter must remove 99.95% (EU standard) and 99.97% (ASME, US DOE) of 0.3 micrometers sized debris from the air that passes through it. This value is used for filter classification because it is close to the most penetrating particle size (MPPS) of approximately 0.2-0.3 μm. (NASA) Particles that are larger or smaller than the MPPS are filtered more efficiently than the rated efficiency, although in some circumstances particles below the MPPS may act as nucleation sites and form particles close to the MPPS. The 2019-nCoV aka coronavirus being 50 to 200 nanometre in size is smaller than the MPPS of HEPA filters, and thus the filter will be able to remove it with at least the rated efficiency. Contrary to common misconception, there is no need to produce special filters for blocking smaller viruses.

Note: HEPA filters are not designed to kill viruses and bacteria and they can stay in the filter as long as they can survive, which can be up to ~9 days for 2019-nCoV in ideal conditions. However, the coronavirus survives best on hard surfaces, and tends to die significantly faster on soft, porous materials such as filter media. This brings us to our next design consideration of the introduction of UV Lights in AC ducts.

[edit] UV Lights in AC ducts

Ultraviolet (UV) radiation is a type of energy produced by the sun and some artificial sources, such as black lights and UV lasers.

The presence of harmful pathogenic microorganisms such as viruses and bacteria in closed occupied spaces is inevitable and these microorganisms can be either killed by controlling the humidity of the space or can be pushed out by an increased (or doubled) ventilation. Since the buildings of today use a recirculating type ventilation system, the chances are that these organisms are either recirculating with the air, stuck to the AC duct walls or in the return air filter.

Introducing UV lamps in the return AC duct and near to the filter can have a significant effect on the indoor air quality by killing harmful viruses and resisting the growth of molds inside ducts. Since lamps prevent the build-up of particles both in the system and in the ducts, they also improve airflow and contribute to energy savings.

[edit] Conclusion

The four above mentioned methods / early design consideration will not only resist the harmful pathogenic microorganisms but will also improve indoor air quality which in turn can reduce operating costs of the AC system.

A new Air Detox mode can be introduced in the HVAC system which has implemented the above-mentioned considerations. The air detox mode could have the following sequence:

  1. High humidity for 20 minutes (This will kill the bacterial and viruses in the space).
  2. Increased ventilation (Will avoid mold making and will regulate the previously increased humidity).
  3. All of the intake fresh air and recirculating air passes through specially design HEPA filters.
  4. UV lights are installed inside the air ducts, preferably close to air filters to kill any residual bacteria and viruses

About the Author: Muhammad is an MEP consultant, LEED Green Associate and has years of experience in the design and development of commercial and residential buildings. He currently lives in Dubai and he loves to write about construction in general and MEP in particular.

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